Search and go to the section where you can block websites, computers, and access during certain times of day. As Router interfaces vary with the maker yet I would suggest you to look for a section with a name such as ‘Access Restrictions’.

Enter the details of the content and the users you’d like to restrict, and save your changes. The documentation of router’s paper would provide you the full details of it.

Be cautious when editing your hosts text file, and always make sure to backup files before you edit them. Good luck!